Machine tools of this kind are disclosed in Japanese Patent Unexamined Laid-Open No. 11722/1980, U.S. Pat. Nos. 3,851,380 issued to M. Kurimoto et al., and 3,509,619 issued to W. Lipp. The machine tool disclosed in the aforementioned Japanese Patent specification has the problem that the capacity to store gang heads is very limited. In each machine tool disclosed in the aforementioned two U.S. patents, a pair of conveyance paths extends from a gang head storage magazine to the sides of the machining unit to take in and out a workpiece, the magazine being mounted in the rear of the machine tool. An access passage extends between these paths and to the portion of the machining unit to which gang heads are mounted. This has introduced the problem that the front portion of the machine is wide.
A machine tool free of these problems is disclosed in U.S. Pat. No. 4,125,932 issued to E. J. kielma et al. In this machine, an arm for simultaneously interchanging two gang heads is mounted between the position at which each gang head is mounted to the machining unit and a standby position that is disposed rearwardly and upwardly of the machining unit. Since the arm rotates about an axis which is inclined at an angle of 45.degree. to the direction of movement of the machining unit, relatively large space is needed to interchange the gang heads simultaneously, or the sleeve of the arm must be long to avoid physical interference between the gang heads and the machining unit during the exchange operation. This increases the time taken to interchange the heads. During the exchange operation, the two gang heads are held only by the arm and so the stability is poor. Hence, the speed at which the arm is rotated must be maintained low. Further, it is necessary that a gang head storage magazine which is located upwardly and rearwardly of the machining unit be disposed sufficiently remotely from the standby position. This has increased the space of the floor occupied by the whole machining system including the frame that supports the storage magazine.